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java Web jsp和servlet的关系

JSP在本质上就是SERVLET，但是两者的创建方式不一样
Servlet完全是JAVA程序代码构成，擅长于流程控制和事务处理，通过Servlet来生成动态网页很不直观
JSP由HTML代码和JSP标签构成，可以方便地编写动态网页
因此在实际应用中采用Servlet来控制业务流程，而采用JSP来生成动态网页

简单的说，SUN首先发展出SERVLET，其功能比较强劲，体系设计也很先进，只是，它输出HTML语句还是采用了老的CGI方式，是一句一句输出，所以，编写和修改HTML非常不方便。后来SUN推出了类似于.NET中的ASP的镶嵌型的JSP，把JSP TAG镶嵌到HTML语句中，这样，就大大简化和方便了网页的设计和修改。新型的网络语言如ASP，PHP，JSP都是镶嵌型的SCRIPT语言。

1 jsp经编译后就变成了Servlet
　　(JSP的本质就是Servlet，JVM只能识别java的类，不能识别JSP的代码,Web容器将JSP的代码编译成JVM能够识别的java类)
2 jsp更擅长表现于页面显示，servlet更擅长于逻辑控制.
3 Servlet中没有内置对象，Jsp中的内置对象都是必须通过HttpServletRequest对象，HttpServletResponse对象以及HttpServlet对象得到.
Jsp是Servlet的一种简化，使用Jsp只需要完成程序员需要输出到客户端的内容，Jsp中的Java脚本如何镶嵌到一个类中，由Jsp容器完成。
而Servlet则是个完整的Java类，这个类的Service方法用于生成对客户端的响应。

通过代码来解释一下：

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>登录测试</title>
</head>
<body>
    <form action="/test/LogInServlet" method="post">
        <table>
            <tr>
                <td>用户名</td>
                <td><input type="text" name="uname" /></td>
            </tr>
            <tr>
                <td>密码</td>
                <td><input type="password" name="psw" /></td>
            </tr>
            <tr>
                <td><input type="submit" value="提交"/></td>
            </tr>
        </table>
    </form>
</body>
</html>

这个jsp文件在编译后会变成java文件login_jsp.java 如下：

这个java类继承了HttpJspBase 而HttpJspBase继承了HttpServlet 也就是说被编译成了一个servlet

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/7.0.82
 * Generated at: 2018-06-26 03:54:34 UTC
 * Note: The last modified time of this file was set to
 *       the last modified time of the source file after
 *       generation to assist with modification tracking.
 */
package org.apache.jsp;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;

public final class login_jsp extends org.apache.jasper.runtime.HttpJspBase
    implements org.apache.jasper.runtime.JspSourceDependent {

  private static final javax.servlet.jsp.JspFactory _jspxFactory =
          javax.servlet.jsp.JspFactory.getDefaultFactory();

  private static java.util.Map<java.lang.String,java.lang.Long> _jspx_dependants;

  private volatile javax.el.ExpressionFactory _el_expressionfactory;
  private volatile org.apache.tomcat.InstanceManager _jsp_instancemanager;

  public java.util.Map<java.lang.String,java.lang.Long> getDependants() {
    return _jspx_dependants;
  }

  public javax.el.ExpressionFactory _jsp_getExpressionFactory() {
    if (_el_expressionfactory == null) {
      synchronized (this) {
        if (_el_expressionfactory == null) {
          _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
        }
      }
    }
    return _el_expressionfactory;
  }

  public org.apache.tomcat.InstanceManager _jsp_getInstanceManager() {
    if (_jsp_instancemanager == null) {
      synchronized (this) {
        if (_jsp_instancemanager == null) {
          _jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());
        }
      }
    }
    return _jsp_instancemanager;
  }
  //servlet中的init
  public void _jspInit() {
  }
  //servlet中的destory
  public void _jspDestroy() {
  }
  //servlet中的service
  public void _jspService(final javax.servlet.http.HttpServletRequest request, final javax.servlet.http.HttpServletResponse response)
        throws java.io.IOException, javax.servlet.ServletException {

    final javax.servlet.jsp.PageContext pageContext;
    javax.servlet.http.HttpSession session = null;
    final javax.servlet.ServletContext application;
    final javax.servlet.ServletConfig config;
    javax.servlet.jsp.JspWriter out = null;
    final java.lang.Object page = this;
    javax.servlet.jsp.JspWriter _jspx_out = null;
    javax.servlet.jsp.PageContext _jspx_page_context = null;


    try {
      response.setContentType("text/html;charset=UTF-8");
      pageContext = _jspxFactory.getPageContext(this, request, response,
                  null, true, 8192, true);
      _jspx_page_context = pageContext;
      application = pageContext.getServletContext();
      config = pageContext.getServletConfig();
      session = pageContext.getSession();
      out = pageContext.getOut();
      _jspx_out = out;
      //我们在js页面上写的html标签
      out.write("
");
      out.write("
");
      out.write("<html>
");
      out.write("<head>
");
      out.write("    <title>登录</title>
");
      out.write("</head>
");
      out.write("<body>
");
      out.write("    <form action="/test/LogInServlet" method="post">
");
      out.write("        <table>
");
      out.write("            <tr>
");
      out.write("                <td>用户名</td>
");
      out.write("                <td><input type="text" name="uname" /></td>
");
      out.write("            </tr>
");
      out.write("            <tr>
");
      out.write("                <td>密码</td>
");
      out.write("                <td><input type="password" name="psw" /></td>
");
      out.write("            </tr>
");
      out.write("            <tr>
");
      out.write("                <td><input type="submit" value="提交"/></td>
");
      out.write("            </tr>
");
      out.write("        </table>
");
      out.write("    </form>
");
      out.write("</body>
");
      out.write("</html>
");
    } catch (java.lang.Throwable t) {
      if (!(t instanceof javax.servlet.jsp.SkipPageException)){
        out = _jspx_out;
        if (out != null && out.getBufferSize() != 0)
          try {
            if (response.isCommitted()) {
              out.flush();
            } else {
              out.clearBuffer();
            }
          } catch (java.io.IOException e) {}
        if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
        else throw new ServletException(t);
      }
    } finally {
      _jspxFactory.releasePageContext(_jspx_page_context);
    }
  }
}

通过代码我们发现 jsp文件在编译的时候会编译成servlet来交给jvm去执行所以我们可以得出一个结论也就是我们文章开头说的：jsp在本质上就是一个servlet。

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原文地址：https://www.cnblogs.com/blazeZzz/p/9228492.html